A stabilization device that interconnects the bone-fixation means, such as pedicle screws, with a longitudinal member, such as a longitudinal spinal rod, extending along at least a portion of a patient's spinal column, is used, for example, for the internal stabilization of the spinal column. In order to securely stabilize the spinal column, it should be possible to connect the bone-fixation means, on the one hand, firmly with the vertebrae and, on the other, with the longitudinal member. Preferably, the head of the bone fixation means and the longitudinal member are connected by a connecting mechanism, which permits the longitudinal member to be fixed with respect to the bone-fixation means at a variety of different angles. Moreover, the connecting mechanism may be detachable, so that the mechanism may be later removed without large openings being formed in the tissue of the spinal column.
Such a connection mechanism between a bone-fixation means and a longitudinal member for the internal stabilization of a vertebrae is known, for example, in U.S. Pat. No. 5,466,237 to Byrd which discloses a bone-fixation screw having a screw head, which is constructed with a spherical shaped lower surface and a convexly shaped upper surface. The spherical shaped lower surface is mounted in a borehole formed in the connecting mechanism, this borehole comprising a concave section, which tapers towards the screw shaft, as a result of which, and together with the screw head, a ball-and-socket type joint connection is established between the bone-fixation screw and the connecting mechanism. This connection is locked by tightening a nut, which can be screwed over an external thread formed on the upper portion of the connecting mechanism. Tightening the nut, forces the nut against the longitudinal member, which is placed in a transverse channel formed in the connecting mechanism for this purpose. The longitudinal member thereafter presses on the convexly-shaped upper surface of the bone-fixation screw, so that, as the nut is tightened, the longitudinal member and the bone-fixation screw are locked in the connecting mechanism.
A bone-fixation screw having a screw head with a spherically shaped lower surface is also disclosed in International Publication No. WO 01/03593 to Frigg (corresponding to U.S. Pat. No. 6,663,635), which discloses that the center of the spherical lower surface and the spherical upper surface can be spaced apart or coincide with respect to the longitudinal axis of the bone-fixation screw.
Another device for connecting a bone-fixation screw with a longitudinal member, which permits limited, separate locking of the longitudinal member and the bone-fixation screw, is disclosed in German Publication No. DE 43 07 576 to Biedermann (which corresponds to U.S. Pat. No. 5,443,467).
One disadvantage of the connecting mechanism in these known devices is that the connecting mechanism must have a generally large, overall height.
A device for connecting a bone-fixation screw with a longitudinal member, which permits a lower overall height due to the configuration of the joint connection and its locking mechanism, is disclosed in International Publication No. WO 01/22893 to Lombardo which discloses a bone-fixation screw having a screw head with a partially lower spherical surface and a top surface having a coaxial, conical cavity for receiving a convexly formed front end of the clamping means which accommodates polyaxially movement and which may be pressed by mounting means against the conical wall of the cavity. The mounting means can be screwed coaxially into the connecting mechanism at the upper end of the connecting mechanism and, when the device is locked, press against the longitudinal member, which is inserted in the transverse channel formed in the connecting mechanism, which in turn, presses on the clamping means. One disadvantage with this device is that, because the convex, front end of the clamping means is pressed into the conical cavity at the screw head of the bone screw, there is a wedging effect which creates a levering force, which aligns the bone screw parallel with the central axis of the connecting mechanism.
Thus, there remains a need for a connecting mechanism that provides a reduced overall height and which overcomes the problems associated with the prior art.